CCAMLR

This paper reports on the trial of daytime fishing during the 1-15 April pre-season extension, in the Patagonian toothfish longline fishery in statistical division 58.5.2.

During the 1-15 April pre-season extension two vessels fished, and all setting was carried out during nautical dusk or night. No seabirds were caught.

Any fishing that occurs in the 1-14 November 2014 post-season extension period, and the planned in the April 2015 extension will be reported to WG-FSA in 2015.
 

During the 2012 CCAMLR toothfish ageing workshop, otoliths from 31 Antarctic toothfish were exchanged between Russia and New Zealand ageing programmes to compare consistency in ages estimated by different readers and using different methods. Both “break and burn” and “bake and embed” techniques were used to prepare otoliths and each were read by an experienced reader from each program. The resulting four-way comparison enabled differences in preparation method to be distinguished from differences in interpretation of otolith banding patterns. Results suggest broad agreement in ages determined by each reader and with each method. However, there were enough inconsistencies in preparation technique and in interpretation of the break and burn preparation method to warrant further coordination and comparisons before merging data. Bake and embed preparations were consistently interpreted by both readers. This experiment highlights the importance of monitoring and comparing ageing protocols within and between fish ageing programmes. The criteria for determining similar age interpretation used here (mean paired age difference statistically equal to zero, overall CV less than 10%, and slope of regression across ages equal to 1) were useful diagnostics in interpreting age comparisons.

Sea ice is recognised as a core driver of both ecosystem dynamics and fishery performance in the Southern Ocean. Sea ice can limit access to fishing grounds, for both commercial fishing and scientific research. Although satellite data are available and have been used for ecosystem monitoring purposes and to inform research plans by CCAMLR, no quantitative summaries relative to fishing access are in use by CCAMLR. We develop a method to spatially and temporally summarise satellite-derived sea ice concentration data in the Southern Ocean, and relate it to data on commercial fishing vessel operations around the Antarctic continental margin. A spatial view is used to characterise the spatial dynamics of sea ice in a target location for a specified period, and a temporal view is used to characterise the inter-annual patterns within a target area. Both can be viewed relative to ice conditions during historical fishing events. The distribution of local ice concentration at the time of each fishing event relative to the weighted ice concentrations within the fishery footprint was used to develop the relationship between sea ice concentration and fishing. Although more than 85% of fishing events occurred in areas with less than 20% sea ice, a threshold of 40–60% sea ice indicated the transition from fished to non-fished conditions. We present a summary of conditions in SSRUs 88.2C-G as a case study to identify and characterise areas with reliable access for the collection of fishery data.

This report summarises the timing, depth, and location of fishing together with the biological aspects and catch of Antarctic toothfish up to an including the 2014 season. In 2014, the Ross Sea slope SSRUs were not constrained by sea ice and catches were evenly distributed across the three SSRUs. As in recent years, the remaining catches came mainly from SSRUs 88.1C, 88.1J, and 88.2H. Unstandardised Antarctic toothfish CPUE in the Ross Sea and Subarea 88.2 fisheries have fluctuated over the past 12 years with a slight decline in the past two years.

Length frequency distributions of Antarctic toothfish in the Ross Sea fishery have continued to be stable in the North and variable on the Shelf. The strong mode of smaller (90–120 cm TL) toothfish present in the Slope fishery from 2010–2013 was not present in the 2014 season, and the size composition in 2014 was more similar to that in the early period of the fishery – the reason for this is unclear. There has been a slight reduction in mean age in SSRU 88.2H, but the data are very uncertain due to the paucity of otolith readings and it is recommended that additional otolith readings for this area are given a high priority. There was a marked increase in the proportion of males in the Ross Sea North fishery from 2001 to 2009, but this has decreased slightly since then. There has been little change in sex ratio in the other areas.

The effect of sea ice is acknowledged as a major influence on fishing operations in high latitudes but not widely understood in detail other than by fishers and specialists in Antarctic fisheries research and management. Using the toothfish fishery in CCAMLR Subarea 88.1 as a case study we provide a method to quantify the effect of sea-ice on fishing.  We discuss the ways in which ice effects fishing operations and the result of these impacts on fishing such as the time required to reach the catch limit and compression of fishing effort due to a reduction in fishing ground extent in years when sea ice is more extensive. The method is a retrospective analysis of past seasons to quantify the level of impact that ice conditions have on fishing.  Graphical summaries of the last 15 austral fishing seasons are included for reference. 

In 2013, the Scientific Committee of CCAMLR could not achieve consensus on a stock status for Antarctic toothfish in Subarea 88.2 SSRUs 88.2C-H and identified several areas for further work. This was presented to WG-SAM in 2014, and found that the models were unable to fit patterns in the recaptures of tagged fish seen in the SSRU 88.2H fishery. The patterns showed a sharp decay rate of a cohort of tagged fish with few being recaptured after more than 3 years at liberty, a steepening of the decay rate over time, and a trend for increasing proportions of tagged fish caught over time.

To help understand the population dynamics which could explain the observed patterns in the tag recapture data we carried out a series of simple simulations in R. This suggested that the observed pattern for that tagging data in SSRU 88.2H was only reproduced in scenarios that included both immigration and emigration,  combined with moderate to high exploitation rates

Within a single area model, emigration can be mimicked by treating it either as a constant biomass of removals or as an additional mortality rate. Both of these approaches were unsuccessful in achieving an adequate fit to the tag data. Clearly, the steep decline in the recapture rates of a cohort of tagged fish through time cannot be explained solely by emigration (a process that includes both tagged and untagged fish), but requires a significant amount of immigration each year to explain the remaining data. Models that include more than one area may be required to model both immigration into SSRU 88.2H and the subsequent emigration back to SSRUs 88.2C–G.

As requested by WG-SAM we have developed models for (i) SSRU 88.2H using tag recaptures from 1 year at liberty; (ii) SSRU 88.2H using tag recaptures from 3 years at liberty; (iii) SSRUs 88.2C-H using tag recaptures from 1 year at liberty, but excluding tags from the south; and (iv) SSRUs 88.2C-H using tag recaptures from 3 years at liberty, but excluding tags from the south. For each model we provide estimates of biomass and yields based on the CCAMLR decision rules. However, we caution that none of these models provide an adequate explanation of the observations available — none of the model captured either the sharp decay in the number of tagged fish recaptured with increasing time at liberty, nor the steepening in that decay over time.

At its 2011 meeting, the Scientific Committee agreed that a time series of relative abundance from a well-designed survey could be a useful input into the Ross Sea stock assessment model. The first survey was completed in February 2012, and the second survey in February 2013. In this paper we provide results of the third survey in the time series. The objectives of this third survey were: (1) To carry out a longline survey to monitor subadult toothfish in the southern Ross Sea (strata A–C) using standardised gear in a standardised manner; and (2) To sample additional experimental stations in an adjacent area to identify areas of high subadult abundance which could be included as strata in future annual surveys.

The 2014 survey was successful in completing all the planned stations. Standardised catch rates for the core strata showed a slight decline across the three surveys but this decline was not significantly different. Age frequency data from the surveys have shown the progression of a cohort from age 7 in 2012 to age 9 in 2014. These results suggest that the surveys are indexing local abundance and will provide a reliable means of monitoring recruitment and estimating recruitment variability. In contrast, standardised commercial catch rates in the core area have been highly variable throughout the history of fishing in the survey area and the age data do not show modal class progression suggesting they are not useful for monitoring recruitment. Stations in the experimental stratum near Ross Island had high catch rates and much larger fish than in the other strata, and warrant future monitoring due to their unique nature. We recommend the survey be continued to provide information on year class strength and an index of local abundance to be incorporated in the stock assessment.

In 2013, the Scientific Committee of CCAMLR could not achieve consensus on a stock status for Antarctic toothfish in the Amundsen Sea Region (ASR - Subarea 88.2 SSRUs 88.2C-H) and identified several areas for further work. This work was carried out and presented to WG-SAM in 2014, who noted that the assessments were unable to fit the patterns in the recaptures of tagged fish seen in SSRU 88.2H. As a consequence, WG-SAM requested further runs be carried out with emigration estimated in the model. However, the models that mimicked emigration in a single area assessment model failed to provide adequate fits to the observed tag recapture data (Mormede et al. 2014b). As an alternative approach, we present the preliminary development of a two-area stock assessment model.

A two-area model is a method for allowing explicit modelling of movement (migration) of fish into an assessment model, allowing the modelling of the populations in the southern ASR (88.2C-G) and the northern ASR (88.2H), as well as modelling the movement of fish between these two areas.

The inclusion of two-areas and migrations into the assessment model has both provided a more plausible explanation of the population structure as well as enabling a better account of the observed patterns in the tag recapture data. However, in this preliminary model some difficulties still remain. While the local biomass in the north and the migration to and from the northern ASR were able to be discerned, the biomass in the southern ASR and the associated proportion of that biomass migrating northward could not be resolved. This was because the parameters that represent the initial biomass in the southern ASR and the proportion of that biomass moving north were highly correlated, with little data to inform estimation of biomass in the southern ASR. As a result, estimates of the biomass and current stock status of the population from these models remains highly uncertain.
This paper is presented for consideration by CCAMLR and may contain unpublished data, analyses, and/or conclusions subject to change. Data in this paper shall not be cited or used for purposes other than the work of the CAMLR Commission, Scientific Committee or their subsidiary bodies without the permission of the originators and/or owners of the data.

Further development of these approaches is recommended, and will need to be further considered by WG-SAM in 2015. Furthermore, because reliable estimation of biomass in the southern ASR is currently not possible due to lack of data, we recommend that the acquisition of additional data to provide an index of abundance for the southern ASR be given a high priority.

Mark-recapture data for Antarctic toothfish from individual seamount features in SSRU 88.2H were analysed to estimate biomass trends on isolated seamounts. Biomass estimates were also calculated for SSRU 88.2H as a whole. The analyses indicate that:

• Fish seldom move among seamounts within the complex, and residence time on particular seamounts declines rapidly over 1–4 years.
• Fishing has occurred on almost every seamount in every year and usually in proportion to the level of tagging on the seamount in the previous year.
• Trends in local biomass estimates for individual seamounts and SSRU 88.2H overall showed a decline in biomass through time with a slight increase in biomass since 2012.
• The pattern in recapture rates of annual cohorts of tagged fish through time in SSRU 88.2H indicates a decrease in the percentage of the population tagged due to the annual immigration of untagged fish, along with catch and emigration of tagged and untagged fish resulting in a decreasing trend in biomass overall.
• Annual immigration also results in a progressive inflation of biomass estimates from mark-recapture data, and therefore biomass estimates are most accurate after 1 year at liberty, but still overestimated due to immigration.

This paper frames a discussion for improving the assessment of toothfish abundance for SSRUs 88.2C–G. We initially provide a characterisation of the fishery and a summary of available tagging and length frequency data. Although 880 tagged fish have been released in this region, only 2 tagged fish have been recaptured. It is likely that the lack of recaptures of tagged fish in this region has been caused primarily by the poor spatial overlap of released tagged fish with subsequent fishing effort.

By drawing on the success of tagging programmes in other CCAMLR fisheries, we develop an approach for improving the spatial overlap of the location of tagged fish and subsequent fishing effort. We identify four main grounds previously fished in SSRUs 88.2C–G. We recommend that in the short term (the next 2–3 years) it be made mandatory for vessels to complete all of their sets inside one or more of these fishing grounds as a condition of fishing in the slope region (SSRUs 88.2C–G). This condition could be relaxed in future years once sufficient tags have been recaptured to carry out an assessment of the stock in this region. We also recommend that the tagging rate in this region be increased to at least 3 tags per tonne in the short term. In addition to improving estimates of abundance a higher tagging rate and more recaptures will also increase information on fish movement within the Amundsen Sea slope which will help reduce uncertainty about the stock identity of toothfish caught in this area.